Why do muscles tighten up and cause pain?

Most think that when a muscle tightens up there is something wrong with that muscles, or in other words that muscles has been overworked. The natural inclination is to relieve the tightness with a stretch or massage.

However, it is important to understand that tightness might be essential. For instance many people complain of tight hamstring. It is important to know that hamstring not only assists in bending of the knee but it also assists in extending of the leg to the rear as well as stabilizing the pelvic area (Lumbo-pelvic-hip-complex). Therefore, if there is a weakness in pelvic stability or extension of the leg to the rear hamstring will compensate by tightening up. In such way, the hamstring acts as a compensating muscle doing a job for someone else. However, the hamstring is not an ideal for those tasks and therefore it is heavily taxed causing you pain and discomfort (in addition to poor joint mechanics in the area).

Another important point to remember is that muscles do not tighten up on their own as a result of physical stress. Instead, the central nervous system (the brain) dictates which muscles will tighten and which will stay active. And here is the most interesting point; the CNS NEVER makes a mistake. Basically the nervous system receives information such as the level of physical stress and the readiness of all muscles participating in the movement. It then makes a decision on the most efficient path to get the job done. Some muscles might be weak, injured, or overworked and therefore they are unlikely to be recruited to handle the stress at hand. In such cases the CNS will recruit another compensating muscles (even if that causes you pain).

Ability to compensate allows us to perform in less than ideal moments. For instance you need to squat to pick up a heave box but your have gotten weaker over the years. The contribution of the weak abs may be insufficient to stabilize your pelvic area and the hamstring will tighten up to compensate for the weakness in the abdominal area. The squat will cause you pain at the hamstring, but, very likely you will be able to pick up the box.